Five examples of building with seaweed
By Jeremy Williams
Last week I reviewed the book The Seaweed Revolution, which I very much enjoyed. It mentions a vast range of uses for seaweed, some in detail, some in passing. One that caught my eye was the use of seaweed as a building material. It only gets a couple of paragraphs, so I bookmarked it to go and look up some examples.
Seaweed has been used as a building material in several different traditions. One of the most impressive is Japanese Shikkui, a lime-based plaster and render that uses seaweed extracts as a glue. As used on Himeji Castle below, it is naturally fire-retardant and anti-bacterial, durable and non-toxic.
That’s not really building with seaweed though, so let’s see if we can do a little better with these homes from Denmark. Seagrass has been used as a thatching material on the island of Læsø for around three hundred years, and necessity was the mother of invention here. They burned all their trees to feed the local saltworks, leaving them short of timber for roofing.
As you might expect from a roof a metre thick, it’s a good insulator. It’s also waterproof, fireproof and rot-proof, and so they last for hundreds of years. Traditionally, roofing was done by the women of the community, who would assemble in groups of 40 or 50 and do a whole house in a day. There are only 36 of these strange and dumpy homes left today.
The articles I read about these Danish roofs all suggested that they are unique. That’s only partly true. Their techniques are unique, but seaweed thatching is also used in the Shandong Province of China, and their roofs are a lot neater and more refined. The tradition goes back a lot longer here too, with the earliest evidence of seaweed roofs dating from the Qin Dynasty some 2,200 years ago.
There are thousands of ‘sea-moss cottages’ in ancient fishing villages across the region. They often use wheat straw and seaweed in alternating layers, sometimes with decoratively weighted old fishing nets over the top to make them more wind-proof. Here are some that have been recently renovated for the tourists:
What about the walls though? Can you use seaweed more structurally? I can’t find any stories of traditional use, which surprises me given how common it is to mix straw or grass into mud bricks. There are some more recent experiments though, with a pioneering entrepreneur in Mexico leading the way.
Omar Vázquez Sánchez lives in Cancun, where vast tides of sargassum seaweed plague the beaches. Hotels pay for the tonnes of stinky weed to be cleared off the beaches, and so Sánchez started a business to turn this waste product into fertiliser. He then began to experiment with it in bricks. The weed is dried, ground and pressed into mud blocks that are cheaper and lower carbon than the conventional alternatives. Each block is about 40% seaweed, so each of the little houses that Sánchez builds have about 20 tonnes of sargassum in the structure.
So far Sánchez’s ‘sargablocks’ have been used in homes locally, but his work has gathered international attention and there is broader interest in the idea. Lots of places have a problem with seaweed tides, and this is a promising low carbon building material.
Can we imagine putting these old and new techniques together to create something new and with mainstream possibilities? A Danish project in Læsø might be the nearest thing. An architect bought and restored one of the traditional cottages, and then built a modern equivalent next door. It uses seaweed in two different ways. One is to pack it into bundled nets to clad the roof and the walls – a neater and more manageable echo of the traditional usage. The other is to use seaweed as insulation, packed between timber panels.
As yet there’s only this one modern seaweed house, built as an experiment and not repeated. It shows us a little of what might be possible, but there is more to come from the idea of seaweed as a building material.
We may yet see insulated structural panels incorporating seaweed. It can be pressed into MDF-style panels or tiles – like this company in the Netherlands is doing with their ‘seawood’ range. It can be made into a translucent leather that can be formed into all kinds of shapes. A company called Seabrick is proposing to use kelp to make buoyant interlocking blocks that form the foundations for floating buildings. Give it a couple of decades, and seaweed as a building material could be unremarkable.
First published in The Earthbound Report.
